This invention relates to the field of exhaust systems for internal combustion engines, and particularly to mufflers of compact configuration and high acoustical efficiency. The advent of equipment powered by small gasoline engines, such for example as snowmobiles, has produced or emphasized the need for reducing the "sound pollution" produced by these engines, within the space and cost limitations unavoidable in such applications.
It has long been known that the noise produced in the operation of internal combustion engines and appearing as the acoustical energy component of the engine exhaust can be reduced or attenuated to almost any level desired, if cost and space constraints are ignored. The art of mufflers is a well-worked one, and numerous sound attenuating expedients are known. A common example is the use of concentric perforated tubes through which the exhaust gases must pass, and its equivalent is found in mufflers where the gases must pass through bodies of material such as glass wool. An alternative method has been to pass the gases through a set of tubes in a re-entrant path whereby destructive interference of the sound energy in selected frequency bands takes place.
Mufflers of the first type are characterized by a tendency to become plugged, and also present so high a loss coefficient that a large area of material for gas flow must be provided, which means that the dimensions of the unit become undesirably large. The wave lengths of the sounds to be attenuated also dictate a muffler structure of undesirably large dimensions, if the alternative method is to be used.
It is customary in muffler design to incorporate within a single housing a plurality of attenuators, sometimes tuned for different acoustical frequencies. One of the most efficient expedients is to use orifices having bell-mouth inlets or adits and diffuser outlets or exits. Here again, the dimensions required for an attenuator capable of effective use to pass the volume of gas exhausted by commercially successful engines is prohibitive. The most use heretofore made of this principal has been to make bell-mouthed the inlets of the perforations in the type of muffler first referred to above.